|
|
|
|
Secure Shell
|
| |
|
| |
Secure Shell or SSH is a network protocol that allows data to be exchanged using a secure channel between two networked devices.
Used primarily on Linux and Unix based systems to access shell accounts, SSH was designed as a replacement for TELNET and other insecure remote shells, which sent information, notably passwords, in plaintext, leaving them open to interception. The encryption used by SSH provides confidentiality and integrity of data over an insecure network, such as the Internet.
DefinitionSSH uses public-key cryptography to authenticate the remote computer and allow the remote computer to authenticate the user, if necessary.
SSH is typically used to log into a remote machine and execute commands, but it also supports tunneling, forwarding arbitrary TCP ports and X11 connections; it can transfer files using the associated SFTP or SCP protocols. SSH uses the client-server protocol.
An SSH server, by default, listens on the standard TCP port 22.
An SSH client program is typically used for establishing connections to an SSH daemon accepting remote connections. Both are commonly present on most modern operating systems, including Mac OS X, Linux, FreeBSD, Solaris and OpenVMS. Proprietary, freeware and open source versions of various levels of complexity and completeness exist.
Uses of SSH 
SSH is most commonly used:
- for port forwarding or tunneling, frequently as an alternative to a full-fledged VPN. In this type of use, a (non-secure) TCP/IP connection of an external application is redirected to the SSH program (client or server), which forwards it to the other SSH party (server or client), which in turn forwards the connection to the desired destination host. The forwarded connection is encrypted and protected on the path between the SSH client and server only. Uses of SSH port forwarding include accessing database servers, email servers, securing X11, rdesktop, Windows Terminal Services and VNC connections or even forwarding Windows file shares. This is primarily useful for tunneling connections through firewalls which would ordinarily block that type of connection, and for encrypting protocols which are not normally encrypted (e.g. VNC);
- for X11-forwarding for through multiple hosts;
- for generally browsing the web through an encrypted proxy connection, using the SSH server as a proxy (with an SSH client that supports dynamic port forwarding);
- for automated remote monitoring and management of servers;
- for securely mounting a directory on the server as a filesystem on the local computer, using the SSH Filesystem;
- as a full-fledged VPN;
SSH architecture
The SSH-2 protocol has a clean internal architecture (defined in RFC 4251) with well-separated layers. These are:
- The transport layer (RFC 4253). This layer handles initial key exchange and server authentication and sets up encryption, compression and integrity verification. It exposes to the upper layer an interface for sending and receiving plaintext packets of up to 32,768 bytes each (more can be allowed by the implementation). The transport layer also arranges for key re-exchange, usually after 1 GB of data has been transferred or after 1 hour has passed, whichever is sooner.
- The user authentication layer (RFC 4252). This layer handles client authentication and provides a number of authentication methods. Authentication is client-driven, a fact commonly misunderstood by users; when one is prompted for a password, it may be the SSH client prompting, not the server. The server merely responds to client's authentication requests. Widely used user authentication methods include the following:
- "password": a method for straightforward password authentication, including a facility allowing a password to be changed. This method is not implemented by all programs.
- "publickey": a method for public key-based authentication, usually supporting at least DSA or RSA keypairs, with other implementations also supporting X.509 certificates.
- "keyboard-interactive" (RFC 4256): a versatile method where the server sends one or more prompts to enter information and the client displays them and sends back responses keyed-in by the user. Used to provide one-time password authentication such as S/Key or SecurID. Used by some OpenSSH configurations when PAM is the underlying host authentication provider to effectively provide password authentication, sometimes leading to inability to log in with a client that supports just the plain "password" authentication method.
- GSSAPI authentication methods which provide an extensible scheme to perform SSH authentication using external mechanisms such as Kerberos 5 or NTLM, providing single sign on capability to SSH sessions. These methods are usually implemented by commercial SSH implementations for use in organizations, though OpenSSH does have a working GSSAPI implementation.
- The connection layer (RFC 4254). This layer defines the concept of channels, channel requests and global requests using which SSH services are provided. A single SSH connection can host multiple channels simultaneously, each transferring data in both directions. Channel requests are used to relay out-of-band channel specific data, such as the changed size of a terminal window or the exit code of a server-side process. The SSH client requests a server-side port to be forwarded using a global request. Standard channel types include:
- "shell" for terminal shells, SFTP and exec requests (including SCP transfers)
- "direct-tcpip" for client-to-server forwarded connections
- "forwarded-tcpip" for server-to-client forwarded connections
This open architecture provides considerable flexibility, allowing SSH to be used for a variety of purposes beyond secure shell. The functionality of the transport layer alone is comparable to TLS; the user authentication layer is highly extensible with custom authentication methods; and the connection layer provides the ability to multiplex many secondary sessions into a single SSH connection, a feature comparable to BEEP and not available in TLS.
Security cautionsSince SSH-1 has inherent design flaws which make it vulnerable to, e.g., man-in-the-middle attacks, it is now generally considered obsolete and should be avoided by explicitly disabling fallback to SSH-1. While most modern servers and clients support SSH-2, some organizations still use software with no support for SSH-2, and thus SSH-1 cannot always be avoided.
In all versions of SSH, it is important to verify unknown public keys before accepting them as valid. Accepting an attacker's public key as a valid public key has the effect of disclosing the transmitted password and allowing man in the middle attacks.
As with any encrypted protocol, SSH can be considered a security risk by companies or governments who do not trust their users and wish to eavesdrop on their communications. Furthermore SSH has built in tunneling features which make it easier for users to achieve passage of large volumes of information or to establish an entry point for unauthorized inward access over a SSH link than with other protocols.
Because of the heavy-weight feature set of the protocol, the ability to use SSH through a firewall may be a serious security risk. In addition to port forwarding, some implementations of SSH directly support Layer2 VPNs, effectively connecting two remote ethernet networks, like they were connected using a switch. Because of these problems, there are attempts to address this issue.
How SSH uses public-key cryptography
First, a pair of cryptographic keys is generated. One is the private key, the other is the public key. As an analogy, they can be thought of as a matching private-key and a public padlock. The public padlock is what is installed on the remote machine and is used by ssh to authenticate users which use the matching private key. As a user of the system, you don’t care who can see or copy the padlock (ie the public key), since only the secret private key fits it. The private key is the part you keep secret inside a secure box that can only be opened with the correct passphrase. When the user wants to access a remote system, he opens the secure box with his passphrase, and uses the private-key to authenticate him with the padlock on the remote computer. Neither the passphrase nor the private key leave the user's machine. However, the user still needs to trust the local machine not to scrape his passphrase or copy his private-key while it's out of the secure box.
See also
External links- (for SSH-2)
- - Home to the most widely used SSH implementation
-
|
| |
|
|
